Digital electronic systems are commonly implemented by combining and interconnecting several different integrated circuit (IC) devices such as processors, memory devices and programmable logic devices. Programmable logic devices (PLDs) are standardized ICs that are readily customizable to perform desired functions. The various IC devices have input/output (I/O) terminals, typically pins or pads on the device, that communicate with one another by way of input and output signals transmitted over a system bus. An I/O power supply VCC_IO provides the necessary power for each device to drive I/O signals over the system bus. Many IC devices, including several types of PLDs, also have a separate internal core power supply VCC_INT that is used for processing signals within the device. The core power supply signal VCC_INT and the I/O power supply signal VCC_IO are often at different voltage levels, and the VCC_IO supply may also be more noisy than the internal core supply VCC_INT. With newer IC devices and I/O standards, the VCC_INT and VCC_IO power supply voltage levels are steadily being lowered.
Advances in process technology have also resulted in a proliferation of different standards, and IC devices may therefore communicate with one another using a variety of I/O standards. In addition to having different VCC_IO requirements, I/O standards may also differ in that they operate using either single-ended, differential, or voltage-referenced input signals. For example, basic TTL (Transistor-Transistor Logic) and LVTTL (low voltage TTL) are single-ended I/O standards in which all input signal levels are taken with respect to circuit ground, e.g., VSS. In differential type standards, such as LVDS (Low Voltage Differential Signaling), differential inputs are required so that there are two I/O terminals (or rails) for each input signal.
In voltage-referenced I/O standards, the I/O structures also use differential amplifier inputs, however one input of each differential amplifier is tied to a common input reference voltage. In many standards, the reference voltage is in the range of 0.7-1.5 V. Thus, unlike single-ended and differential type I/O standards, voltage-referenced I/O standards require that a separate input reference voltage be provided to an IC device. Existing voltage referenced I/O standards include the HSTL (High Speed Transceiver Logic) and SSTL (Stub Series Terminated Logic) standards commonly used for high-speed memory, as well as the GTL (Gunning Transceiver Logic) standard used for high speed, low power backplane communications.
In some logic devices, particularly PLDs, the I/O terminals are programmable and the device supports operation according to several different I/O standards. In these devices, the same I/O terminals receive input signals regardless of the I/O standard being used. A dedicated I/O terminal may be used to receive the input reference voltage necessary for operation in a voltage-referenced I/O standard. However, since some I/O standards do not use an externally supplied input reference, a switch circuit is needed to pass the reference voltage to I/O buffer circuits for standards that require it and block that transmission for standards that do not. Unfortunately, existing switch circuits are often not capable of adequately passing the reference voltage to the input buffers since the VCC_IO supply powering the switch circuit for a given voltage-referenced standard is relatively low (e.g., 1.5 V or less). As a result, one or more transistors in the switch circuit may not turn on sufficiently due to the low voltage supply levels. When such inadequate powering of the transistors occurs, the level of the reference voltage output by the switch circuit is too low. This is a significant concern in many PLDs, since low VCC_IO levels are becoming more common in advanced process technologies. Consequently, there is a need for a more effective on/off input reference voltage switch circuit suitable for use in devices using standards having a low VCC_IO power supply.